1. Field of the Invention
The present invention relates to a nitride semiconductor laser device having an active layer made of gallium nitride semiconductor, particularly to a nitride semiconductor laser device having a current blocking layer made of InxAlyGa1-x-yN (0≦x≦0.1, 0.5≦y≦1, 0.5≦x+y≦1).
2. Prior Art
Gallium nitride semiconductor laser is capable of oscillating in a wide range of wavelengths from ultraviolet to red light, and is expected to have variety of applications such as light sources for optical disk system, laser printer and optical network. In the gallium nitride semiconductor laser of the prior art, it has been a common practice to employ ridge waveguide structure that has stripe-shaped ridge formed on a cladding layer or the like located on an active layer, for the stripe structure formed to control the horizontal transverse oscillation mode.
However, since mechanical strength of the ridge waveguide structure is weak at the ridge, defects are likely to occur particularly when mounted face down. Also because variations are caused in the threshold current and/or beam shape depending on the dimensions of the ridge, it is difficult to manufacture laser devices of uniform characteristics. For this reason, attempts have been made to control the horizontal transverse oscillation mode by forming an insulation layer (current blocking layer) having a stripe-shaped window as a current path, over the active layer, instead of the ridge waveguide structure.
For example, Japanese Unexamined Patent Publication (Kokai) No. 2002-314203 proposes a gallium nitride semiconductor laser having a current blocking layer formed from AlN in a p-type optical guide layer of the active layer. The stripe structure of this laser is made as follows. First, the current blocking layer made of AlN is formed on a device, on which layers up to the p-type optical guide layer have been formed, at a temperature from 400 to 600° C. in a reaction furnace of MOCVD apparatus. After taking out the wafer from the reaction furnace, stripe-shaped window is formed by photolithography process using an alkaline etching solution. Then the wafer is returned into the reaction furnace of the MOCVD apparatus where p-type optical guide layer is grown so as to fill in the window of the current blocking layer, and p-type cladding layer and other layers are formed successively.